Tire tread provided with incisions

ABSTRACT

Tire tread including a pattern formed by at two ridges of mean width L, which include a plurality of incisions whose traces on the contact surface extend between two points of intersection A and B, the segment AB making an angle with the transverse direction of the tread at most equal to 40°, the tread wherein each incision, of mean width E, includes a succession of incision portions, some of said incision portions having, on any surface parallel to the contact surface in the new condition and located between said surface in the new condition and ⅔ of the maximum depth of the incision, traces that make an average angle β at most equal to 15° with the longitudinal direction of the tread, said portions having a total length Lt which is at least equal to one-fifth of the ridge width; and in that the incision portions whose traces make an average angle β are provided on their opposite walls with relief elements of amplitude K designed to cooperate with one another to block relative movements between one incision wall and the opposite wall, the amplitude K of said relief elements being between 4 and 10 times the mean width E of the incision.

The present application is a continuation of U.S. Ser. No. 10/753,418filed on Jan. 9, 2004, now U.S. Pat. No. 7,201,195 issued Apr. 10, 2007,and which is a continuation of International Application Ser. No.PCT/EP02/07003, filed Jun. 25, 2002, which is incorporated by reference.

FIELD OF THE INVENTION

The invention concerns the treads of tires intended for fitting to thedriving axle of vehicles, and more particularly heavy vehicles.

BACKGROUND OF INVENTION

Such a tread is generally provided with a tread pattern formed of reliefelements, ridges and/or blocks, separated from one another in thetransverse and/or circumferential direction by circumferential and/ortransverse grooves. Depending on the axle on which a tire is fitted, itstread is provided with a pattern comprising more blocks or more ridgesof generally circumferential orientation. In the case of a driving axle,i.e. an axle to which a driving or braking torque is transmitted, it iscustomary to produce, rather, patterns that comprise a plurality ofblocks delimited by both circumferential and transverse grooves. Themajor disadvantages of this type of pattern are that the pliability ofthe tread is increased (reduction of the shear rigidity under a forcetangential to the surface of the relief elements passing through thecontact zone) and that the noise emission during rolling increases withthe amount of grooving (ratio between the areas of grooves and the totalarea of the tread).

Another possibility consists in producing a pattern formed of ridgeswith generally circumferential orientation (i.e. which can zig-zageither side of that direction) and providing these ridges with aplurality of incisions whose width is small compared to the width of thegrooves (as a general rule, the average width of the incisions is atmost 2 mm); these incisions may or may not be oblique relative to thetransverse direction of the tire, and may extend through part or all ofthe useful thickness of the tread (by definition, the useful thicknesscorresponds to the thickness of tread that a user can use while stillcomplying with the legal prescriptions in force). Although the presenceof these incisions favors road grip due to the presence of the numerousedges that they form on the rolling surface of the ridges, an increaseof their number also reduces rigidity, which is detrimental to thetire's other performance characteristics, this reduction of rigiditybeing in part associated with the ability of the opposite rubber wallsthat delimit each incision to slide relative to one another duringpassage through the road contact zone.

A solution was proposed in particular by the applicant in patent EP768958, which describes new forms of incisions that substantially reducethe relative sliding between opposite walls, and according to which saidwalls each have a relief surface formed of projections and concavitiesarranged on either side of a mean surface, a projection being completelysurrounded by concavities and vice-versa, such that the wall surfacescooperate together to restrict the relative movements between saidwalls.

In general these incisions are arranged so as to form rubber edgesorientated essentially transversely so as to have an effect on the gripof the tire under a driving or braking torque when rolling in a straightline. It has been found that under forces tangential to the contactsurface of ridges provided with such incisions, although to be sure thewalls are blocked against the opposite walls, this blocking does notbecome effective instantaneously. During noise measurement tests onheavy vehicles that consisted in applying a driving torque to thedriving axle, the applicant found that the larger was the number ofincisions of the type described in patent EP 768958, the moreperceptible became the noise recorded as the vehicle passed. In fact,there are directions in which there is no blocking between the walls.

The need exists for a tire tread pattern formed essentially ofcircumferential ridges provided with a plurality of incisions havingmean width less than 2 mm, which create a large number and a largelength of edges, said incisions delimiting a plurality of rubberelements whose opposite walls are mutually blocked against one anotherwith virtually instantaneous effect (i.e. in a time which is veryappreciably reduced or zero).

SUMMARY OF THE INVENTION

To that end, the invention proposes a tread for tires which comprises apattern formed by at least two grooves of generally circumferentialorientation, these grooves delimiting at least two ridges eachcomprising a contact face intended to come in contact with the road andtwo lateral faces, said lateral faces intersecting the contact face toform edges, at least one of said ridges, of mean width L, having aplurality of incisions that open onto the contact face and onto the twolateral faces of the ridge, such that the trace of each incision on thecontact surface when the tire is new extends between two intersectionpoints A and B with the edges of the ridge, and the direction of thesegment AB makes an angle α with the transverse direction of the treadat most equal to 40, said tread being characterized in that:

-   -   each incision, of mean width E, comprises between one lateral        face of the ridge and its other lateral face a succession of        incision portions, some of said incision portions, over at least        a total height He equal to half the maximum depth Hi of the        incision and for any surface parallel to the contact surface in        the new condition and taken within this height He, having traces        which make an average angle β at most equal to 15 relative to        the longitudinal direction of the tread, and in projection on        this longitudinal direction, said portions having a total length        Lt which is at least equal to one-fifth of the width L of the        ridge; and in that    -   the incision portions whose traces make an average angle β are        provided on opposite walls with relief elements of amplitude K        designed to cooperate during passage through the road contact        zone, to block relative movements between one wall of the        incision and the opposite wall in the direction of the tread's        thickness and in the longitudinal direction of the tread, the        amplitude K of said relief elements being between 4 and 10 times        the mean width E of the incision.

The total length Lt is equal to the sum of the lengths of theprojections on the longitudinal direction Y of the incision portionswhich make an average angle of at most 15° with the longitudinaldirection Y. This length Lt can be different from one section plane toanother section plane, which is equivalent to the fact that from onesection plane to another the trace of the incision on the rollingsurface evolves.

The mean width E of an incision is defined as the average distanceseparating its opposite walls.

To obtain the result desired, it is essential to achieve the combinationof a sufficient length of incision portions that make a small angle withthe longitudinal direction of the tread (corresponding to thecircumferential direction when the tread is part of a tire) and thepresence of relief elements on those portions to achieve blocking of thewalls of said portions in all the directions contained on the surface ofsaid portions, and this over at least a total height He equal to atleast two-thirds of the maximum height Hi of the incision. Of course,this height He can be taken as the sum of the heights of severalunconnected portions which satisfy the conditions of the inventionspecified above, namely the conditions of angle and length.

Thus, when passing through the ground contact zone, the ridges providedwith incisions according to the invention can be likened to mechanicallycontinuous ridges (i.e. without incisions), since the opposite walls ofthe incisions are at least locally blocked against one another toprevent any relative movement in directions parallel to said walls (i.e.other than perpendicular to these walls).

Preferably:

-   -   the total length Lt is at least equal to one-third of the width        L of the ridge;    -   each incision portion making an average angle β at most equal to        15° has, in projection on the longitudinal direction of the        tread, a length between one-quarter and half the mean distance        between two incisions;    -   to improve the blocking still further, each incision portion        making an average angle β at most equal to 15 has, in projection        on the longitudinal direction of the tread, a length at least        equal to one-fifth the width of the ridge;    -   the walls of incisions making an average angle β at most equal        to 15° are provided with relief elements over at least 50% of        the maximum depth of the incision from the rolling surface in        the new condition; advantageously, these relief elements        (concave or raised) are distributed along at least three lines        whose level is essentially parallel to the rolling surface;    -   the average angle β is at most equal to 5°, to achieve even more        effective blocking of movements in the longitudinal direction of        the incision walls;    -   all the incision portions are provided with relief elements on        their walls, these relief elements being designed to cooperate        between themselves to block all movements along directions        parallel to said walls.

The tread according to the invention functions almost identically to thesame tread provided with a pattern formed of ridges, while having anumber and total length of edges in the contact zone which confer uponthe tread a greatly superior grip performance. In effect, all relativemovements of one wall relative to the wall opposite, except for theirmoving apart, are very markedly reduced or even prevented entirely,whether a rib provided with such incisions is subjected in the contactzone to tangential longitudinal (circumferential) or transverse forces,or to a combination of such forces.

Besides, it has been found that a tread pattern according to theinvention does not suffer additional wear and in particular that it doesnot suffer irregular wear (i.e. the wear observed is essentially uniformover the entire contact surface of the ridges).

The relief elements (projections and concavities) can have anygeometrical shape, but it is preferable for these shapes to be similarso as to allow even more effective cooperation. It is of course possibleto form only projections on one wall delimiting an incision and onlyconcavities complementary to the projections on the opposite wall.

To achieve still better blocking, it is advantageous to distribute overthe entire width of the ridges the presence of incision portions thatmake average angles β at most equal to 15°. To do this, and if it isnoted that:

-   -   Le: maximum distance separating the incision portions which        comprise relief elements and are located closest to the lateral        faces of each ridge, and    -   Li: maximum distance separating the incision portions which        comprise relief elements and are located closest to the median        portion of each ridge,        it is advantageous for these distances to satisfy the following        relationships:        ⅓≦Le/L≦⅔        ¼≦Li/L

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention emerge from thedescription given below with reference to the attached drawings, whichshow embodiments of the object of the invention presented asnon-limiting examples.

In the drawings:

FIG. 1, illustrating a first variant, is a plan view of a ridge in atread of a tire according to the invention;

FIG. 2 shows a section of an incision in the ridge shown in FIG. 1,along the line II-II;

FIG. 3 shows a plan view of a tread ridge in a second variant accordingto the invention;

FIG. 4 shows a section of part of an incision in the ridge shown in FIG.3, along the line IV-IV;

FIG. 5 shows a section of part of the incision in the ridge shown inFIG. 3, along the line V-V;

FIG. 6 shows a general view of an element for molding an incisionaccording to the invention;

FIG. 7 shows another variant of an incision according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a partial view of a ridge 1 of a tread for a heavy vehicletire of size 315/80 R22.5, comprising a total of eight ridges delimitedby grooves with a mean depth of 15 mm, orientated in the circumferentialdirection of the tread (represented by the direction Y in FIG. 1).

The ridge 1 has a contact face 2 intended to come in contact with theground during rolling, and two lateral faces 3 and 4 intersecting thecontact face 2 along two edges 5 and 6. The ridge 1 is provided with aplurality of incisions 7 opening onto each lateral surface; viewed onthe rolling surface of the tire in the new condition, each incision 7intersects said surface 2 along a line extending between points A and Blocated on the edges 5 and 6 of the ridge. The average angle of thetrace of the incision on the rolling surface corresponds to the angle αbetween the segment AB and the transverse direction of the tread (markedas direction X in the figure); this angle is in the present case equalto 7°. Each incision in a given ridge has a mean width E of 0.6 mm andvirtually constant depth over its length (between points A and B), equalto the depth of the grooves that delimit the ridge 1.

Each incision 7 extends between the lateral face 5 and the other lateralface 6 of the ridge 1 and is formed by a succession of straight incisionportions 70, 71, 72, 73, 74 whose intersections with the rolling surfacecan be seen in FIG. 1.

Two portions 71, 72 of the incision 7 have traces on the rolling surfacewhich make angles with the longitudinal direction of the tread (markedas direction Y in the figure) of β2 and β2 respectively, which are inthis case both equal to 7°. These two incision portions 71 and 72 havelengths L1 and L2 both equal to 5 mm, the sum Lt of these lengthsprojected on the longitudinal direction Y being in this case equal toone-third of the width L of the ridge.

The two incision portions 71 and 72 are in addition provided on thewalls delimiting them with a plurality of projecting and concave reliefelements rather similar to those described in the patent U.S. Pat. No.5,783,002. FIG. 2, which is a section along the line II-II in FIG. 1,shows the incision 71 of mean width E and depth Hi. Each wall of theincision 71 has concave and projecting relief elements with a maximumamplitude K equal to 3.3 mm in the present case. In addition, the wallsof this incision 71 have concave and projecting relief elements over atotal height He in this case equal to Hi. Said relief elements aredistributed at several levels between the rolling surface and the bottomof the incision: in practice, and viewed in the direction of theincision's depth Hi, there are preferably at least three rows of reliefelements.

In the example presented, the portions 70, 73 and 74 of the incision arealso straight and intersect the rolling surface along rectilinear traceswhich make the same angle α equal to 7. Furthermore, these portions alsohave a plurality of relief elements similar to those arranged on theportions 71 and 72. In the example described, the direction of rotationis opposite to the direction Y in FIG. 1.

Globally, the incision 1 is perpendicular to the rolling surface in thenew condition, i.e. each portion 70, 71, 72, 73, 74 is on averageperpendicular to said surface.

As a variant, each incision may be inclined at an angle whose absolutevalue is at most equal to 15° (relative to the direction perpendicularto the rolling surface); the incisions in a given ridge can be inclinedin alternation at a positive and at a negative angle.

Table I below presents a comparison of various patterns having ridges(case 1 to case 4):

-   -   without incisions (case 1);    -   with flat incisions having a rectilinear trace on the rolling        surface (case 2);    -   with flat incisions having a plurality of relief elements as        described in U.S. Pat. No. 5,783,002 (case 3);    -   with incisions according to the present invention (case 4).

In this last case 4, the tire comprises eight ridges each provided witha plurality of incisions such as those shown in FIG. 1. The incisions ofthe ridge at the shoulder differ from those in the other ridges in that,since the shoulder ridge is wider, these incisions are provided with anadditional undulation of larger amplitude

To compare these tread patterns (Table I), a rigidity in longitudinalshear (along the direction Y in FIG. 1) is calculated for each of them,equal to the ratio between the longitudinal force exerted by the groundon each pattern and the shear deformation imposed on said pattern. Apliability is also calculated, which is defined as the ratio between therigidity in longitudinal shear and the product obtained when the shearmodulus of the rubber is multiplied by the pattern area in contact withthe ground.

A grooving proportion of each tread pattern is defined as the ratiobetween the sum of the areas of the incisions on a ridge in the newcondition and the total area of said ridge.

TABLE I Calculated values Rigidity in longitudinal shear, Pliability,Grooving, in daN/mm in % in % Case 1 44.30 98.66 0 Case 2 24.64 56.58 3Case 3 21.79 50.55 4 Case 4 26.59 62.57 5.33

It is clear that with the pattern according to the invention (case 4)the grooving proportion and length of edges can be increased while alsoincreasing the rigidity in longitudinal shear, compared with what isobtained with patterns having incisions not according to the invention,without on the other hand producing too great a pliability.

Measurements have also been made on tires provided with the samepatterns (cases 2, 3 and 4) and a pattern having ridges provided withbridged rectilinear incisions (case 5), i.e. ones whose opposite wallsare connected by bridges of rubber. These measurements are shown inTable II below:

TABLE II Measurements on tires Mean rigidity in Mean groovinglongitudinal shear per proportion per ridge, in daN/% ridge, in % Case 250 3 Case 3 55 3 Case 4 60 5.33 Case 5 55 2.9

It is clear that the tread pattern provided with incisions according tothe invention (case 4) retains a higher rigidity (expressed in daN per %deformation) than a pattern provided with incisions having bridges,while offering greater grooving regardless of the wear level.

FIG. 3 shows another variant according to the invention of a ridge 10 ofa tread comprising a plurality of incisions 700. Viewed on the rollingsurface 20, each incision has a trace which extends between two points Aand B of intersection with each edge of the ridge. This incision hasseveral successive incision portions going from A towards B, namely: AC,CD, DE, EF, FG, GH, HI, IJ, JB. The incision portions AC, DE, FG, HI arealigned with the transverse direction X and do not have relief elementson the walls which delimit them.

The portions CD, EF, GH, IJ each make an average angle β whose absolutevalue is 5° relative to the longitudinal direction Y (the angle beingthat of the segment connecting the points at the ends of each of saidincision portions). All these incision portions are provided with aplurality of relief elements throughout their depth (as indicatedsymbolically by the undulating lines 8 on the rolling surface, as shownin FIG. 3).

To improve the blocking of the walls of longer incision portions, themaximum amplitude K of the relief elements is located towards the medianarea of each such portion (in the example shown, the amplitude of therelief elements increases progressively from each end towards the medianarea, as indicated schematically by the dotted lines).

It is advantageous for the incision portions inclined at an angle βsmaller than 15° relative to the longitudinal direction of the tread, tobe distributed over the width of the ridge in such manner that theblocking of the incision's opposite walls is as evenly distributed aspossible over the full width of the ridge. In the present case, themaximum width Le separating the incision portions comprising reliefelements and located closest to the lateral faces of each ridge(corresponding to portions CD and IJ and more particularly to thedistance between the points C and J), is equal to ⅔ of the width L ofthe ridge 10, while the maximum width Li separating the incisionportions with relief elements located closest to the median part of theridge, is equal to one-third of the width L.

Advantageously, the incision portions without any relief elements areinclined at a positive and a negative angle relative to a median planeperpendicular to the rolling surface and passing through the point A,this inclination being equal to at most 15°.

In the present case, the incision portions DE and HI without reliefelements are inclined relative to the plane oriented perpendicular tothe rolling surface of the tread in the new condition, as shown in FIG.4. FIG. 4, which is a section along the line IV-IV in FIG. 3, shows thatthe trace of the incision portion HI is inclined at an angle γ₁ relativeto a perpendicular to the rolling surface passing through the point ofintersection of that portion with said surface. On the other hand, theincision portion FG located on the other side of the segment AB relativeto the portions DE and HI, is inclined at an angle γ₂ of the same valuebut opposite sign to the angle γ₁, as shown in FIG. 5, which is asection along the line V-V of said incision portion FG.

If L′ and L″ denote the distances of the incision points furthest awayin the longitudinal direction from the notional line passing through thepoints A and B and located on either side of this line, various casesare possible. In the example described, L′ and L″ are essentially equalto one another and to one-quarter of the width L of the ridge.

For better functional equilibrium of the tread and better blocking ofthe incision walls, it is advantageous for the lengths L′ and L″ to beweighted according to the incision portions orientated virtually in thelongitudinal direction: in the present case, it would be preferable tohave L″ essentially equal to twice the length of L′.

FIGS. 1 and 3 show ridges provided with incisions according to theinvention, with a preferential orientation on said ridges: of course,the same effect can be obtained with a tread pattern in which aplurality of incisions are produced in each ridge, the trace of eachincision being essentially symmetrical relative to the line connectingthe points of intersection of said incision with the edges of saidridge.

The relief elements may be in the form of grooves and ridges designed tocooperate with one another to ensure the blocking of any movement oncethe walls of incisions provided with such elements are in contact withone another. The not necessarily rectilinear grooves and ridges areformed on opposite walls along average directions which may or may notbe perpendicular to the rolling surface.

FIG. 6 shows a variant of a molding element 101 in the form of a lamelladesigned for fitting in a mould for molding an incision according to theinvention in a tire tread. This lamella 101 comprises two oblique parts102 (i.e. ones designed to mould the parts of incisions that form anangle different from zero with respect to the direction perpendicular tothe rolling direction and radial parts 103 (designed to mould parts ofthe incision perpendicular to the rolling surface) intercalated.

All these parts of the lamella have the same mean thickness. The totalheight of the lamella is denoted Hi. All the parts of the lamella haveconcave 104 and raised 105 elements arranged in alternation, whoseamplitude is between 4 and 10 times the average thickness of thelamella.

Some of the radial parts 106 are designed to mould incision portionsthat form an angle equal to 0 with respect to the longitudinal directionof the tread (the other parts being in this case perpendicular to saiddirection).

The lamella 101 described is designed for molding an incision whoseintersection with the rolling surface has a crenellated geometry, theamplitude of the crenellation increasing progressively with depth (andtherefore as the tread becomes more worn). Beyond a certain depth andover at least a residual depth He which is at least equal to half thetotal height Hi, the radial parts 106 which mould incision portionsangled at 0° relative to the longitudinal direction, viewed inprojection on said direction, have a total length Lt at least equal toone-fifth of the length L of the lamella (total length means the sum ofthe lengths in the longitudinal direction of said radial parts).

Another interesting variant of this last lamella corresponds to theschematic representation of FIG. 7. FIG. 7 represents a rubber element201 of a tread provided with an incision 202 molded with a lamellafairly similar to that described above.

The particular feature of the incision 202, whose total height is Hi, isthat it forms on the rolling surface a geometrical trace 203 which, aswear proceeds, evolves from a crenellated shape with the crenellationlocated on a first side relative to the segment joining the ends A and Bof said incision, to another crenellated shape with the crenellationlocated on the opposite site relative to the same segment, passingthrough an intermediate position where the geometrical trace isessentially rectilinear (about half-way down the incision in the caseillustrated). The shape of the geometrical trace only begins varyingafter a depth He1 and continues monotonically until a depthcorresponding to a level located a distance He2 from the bottom of theincision.

At least the opposite walls of the incision portions 204 aligned withthe longitudinal direction of the rubber element comprise concavitiesand projections (not shown in this representation), such that the sum ofthe lengths projected on the longitudinal direction of said portions andon the height He equal to the sum of the heights He1 and He2, is atleast equal to one-fifth of the width L of the rubber element 201. Here,the height He is equal to half the total height Hi of the incision.

The invention is not limited to the examples described and illustrated,and various modifications can be made without going beyond its scope. Inparticular, tires for passenger cars are also concerned, as well astires for non-driving axles of heavy vehicles.

1. A tread for a tire, comprising a tread pattern formed by at least twogrooves of generally circumferential orientation, these groovesdelimiting at least two ridges each comprising a contact face intendedto come in contact with the road and two lateral faces, these lateralfaces intersecting the contact face to form edges, at least one of saidridges, of mean width L, comprising a plurality of incisions withmaximum depth Hi which, in the new condition, open onto the contact faceand onto the two lateral faces of the ridge, the trace of each incisionon the contact surface extending between two points of intersection Aand B with the edges of the ridge, the segment AB making an angle α withthe transverse direction of the tread at most equal to 40°, said treadbeing characterized in that: each incision of mean width E, comprises,from one lateral face of the ridge to its other lateral face, asuccession of incision portions, some of these incision portions, overat least a height He equal to half the maximum depth Hi of the incisionand for any surface parallel to the contact surface in the new conditionover said height He, having traces that make, with the longitudinaldirection Y of the tread, an average angle β at most equal to 15°, saidsome incision portions, viewed in projection on this longitudinaldirection Y, having a total length Lt at least equal to one-fifth of thewidth L of the ridge; and in that the incision portions whose tracesmake an average angle β are provided on their opposite walls with reliefelements of amplitude K designed to cooperate with one another, duringpassage through the road contact zone, to block the relative movementsbetween one incision wall and the opposite wall in the direction of thethickness of the tread and in the longitudinal direction of the tread,the amplitude K of said relief elements being between 4 and 10 times themean width E of the incision; each of other incision portions of saidsuccession of incision portions of the incision extending into the treadin a direction forming an average inclination γ smaller than 15° with aplane that is oriented perpendicular to the contact surface of thetread, wherein the respective inclinations γ of the other incisionportions are opposite one another; wherein each of the other incisionportions make an angle with the longitudinal direction which isdifferent than the angle β.